The present invention relates generally, as indicated, to centrifugal blowers and, more particularly, to a means for reducing the noise of a centrifugal blower.
Centrifugal blowers or centrifugal fans (the terms blower and fan being used interchangeably herein) are, of course, well known devices for blowing air, or as desired, other fluids. As is described in the Marks' STANDARD HANDBOOK FOR MECHANICAL ENGINEERS, McGraw-Hill Book Company, New York, 1967, at pages 14-72 through 14-78, a centrifugal fan has a fan wheel and a casing or housing with a cutoff, an air inlet and an air outlet. The fan wheel is of generally cylindrical configuration having blades facing forwad or backward relative to the direction of rotation thereof about the axis of the cylinder. The casing typically is generally spiral in shape to collect the air delivered from the fan wheel and to conduct the same in a spiral flow pattern to the outlet.
An example of a prior art centrifugal blower is illustrated in FIG. 1 of this application. The blower 1 includes a cylindrical fan wheel 2 having a radial center 3 along its axis and a spiral shape blower housing 4 with a blower outlet 5 at one end thereof. The air inlet 6 to the blower 1 is in a side wall 7 to permit air flow into the center of the fan wheel.
The spiral blower housing 4 has an approximate center of curvature or radial center, i.e. the theoretical center of the spiral thereof, not shown, but approximately falling in the vicinity of the radial center 3 of the fan wheel 2. The blower housing 4 has two wall portions which lead generally to the blower outlet 5; one of those wall portions 8 is that part of the spiral shape blower housing that is relatively far or remote from the spiral center, and the other wall portion 9 may be considered the involute wall portion, which generally curves inwardly along the track of the spiral toward the center thereof. Typically the fan wheel 2 is positioned such that it is relatively near the involute wall surface 9, but relatively far from the remote wall portion 8. With the fan wheel 2 so positioned, there is a generally annular spirally expanding flow path along which air may be blown by the rotating fan wheel 2 toward the blower outlet 5 for discharge from the latter in a generally linear flow direction or flow path. Linear is used herein to indicate a non-spirally confined flow path and may be a divergent one, as the air blown through the blower outlet 5 may diverge or expand upon so leaving.
In the past a cutoff 10 has been provided at the end of the involute wall portion 9 relatively proximate or extending into the blower outlet 5. At least part of such cutoff 10 somewhat continues the involute curve of the spiral. According to the above-mentioned text, Marks', such a cutoff is required in a centrifugal blower to provide a transition from the spiral air flow occurring in the housing to the relatively straight-line discharge air flow through the blower outlet 5. In providing such transition, the typical cutoff, such as the cutoff 10 illustrated in FIG. 1, tends to cut off or to impede air flow through the clearance area between the cutoff and the fan wheel 2, which would ordinarily rotate in a counter-clockwise direction relative to the illustration of FIG. 1. As is also described in such text, typically the blower outlet 5 would connect with a duct having a greater height dimension that the height of the blower outlet 5, such greater height dimension being, for example, equal to the distance between the wall portion 8 and the radial center 3 of FIG. 1; and in such case, the cutoff 10 and the throat of restricted flow area section 10' formed by the illustrated wall portion 10" associated with the cutoff 10 and generally parallel with the wall portion 8 clearly impedes the flow of air into the outlet duct, not shown.